Attempts have been made to add fillers into resins for improvements in the mechanical strength, dimensional stability and thermal resistance etc. of the resins. Although a glass fiber is widely used as a reinforcement for resins, it is difficult to obtain a transparent material from the resin and the glass fiber due to the difference in reflective index between the glass fiber and the resin and the size of the glass fiber and is thus difficult to use the glass fiber in a material, such as an automotive window material, for which transparency is required.
As a solution to this problem, there is a demand for a filler having a smaller reflective index difference to a resin and being finer in particle size and capable of being dispersed uniformly in the resin. In view of the fact that aluminum oxide has a small reflective index difference to a polycarbonate resin, it is expected that a composition of the polycarbonate resin and aluminum oxide can be provided with good transparency.
Various proposals have been made on the use of aluminum oxide as a filler in resins. For example, Patent Document 1 teaches a resin composition in which a resin is melt-mixed by a mixer with needle-like boehmite particles or needle-like alumina particles having a particle length of 1 to 10 μm and an aspect ratio of 40 to 70. This resin composition cannot however show sufficient transparency as the needle-shaped particles used has a much larger size than the wavelength of visible light and insufficient dispersibility to the resin.
Patent Documents 2 and 3 teach resin compositions containing needle-like aluminum oxide nanoparticles. More specifically, Patent Document 2 teaches a polycarbonate resin composition containing aluminum oxide nanoparticles and having good transparency and mechanical properties. According to studies made by the present inventors, however, it can hardly be said that the resin composition of Patent Document 2 is suitable for use as the material where low linear thermal expansion coefficient is required such as automotive window material since the aluminum oxide particles added are small in amount and insufficient in dispersibility.
Patent Document 3 teaches a resin composition in which needle-like boehmite particles without surface treatment are dispersed in a resin. According to studies made by the present inventors, it has however been found that: in order for the particles without substantial surface treatment to be dispersed in the resin so as not to cause aggregation of the particles, the resin is limited to that having a strong polar group or groups in its polymer chain structure (e.g. polyamide, thermoplastic polyurethane); and the aggregation of the particles cannot be avoided in the case of the polycarbonate resin with relatively weak polarity. It has further been found that the mechanical properties of the resin composition significantly deteriorate as the molecular weight of the polycarbonate resin becomes decreased under the catalysis of the boehmite.
There are known some techniques to modify aluminum oxide for improvement in dispersibility. (See e.g. Patent Documents 4 and 5.) In the dispersibility improvement technique of Patent Document 4, the aluminum oxide is surface treated with a silane coupling agent. It has been confirmed that, in the case of treating the aluminum oxide with a trifunctional silane coupling agent as taught in the above document and blending the treated aluminum oxide in the resin, the aggregation of the aluminum oxide in the resin composition cannot be avoided so that the transparency of the resin composition becomes impaired. It has been further confirmed by the present inventors that, in the case of treating the aluminum oxide with a silane coupling agent having a basic or acid functional group or groups and bending the treated aluminum oxide in a polycarbonate resin, the mechanical properties of the resin composition becomes impaired due to the decrease in the molecular weight of the polycarbonate resin.
In the technique of Patent Document 5, the aluminum oxide is treated with a sulfonic acid. It has been found by the present inventors that, in the case of treating the aluminum oxide with a sulfonic acid having a carbon number of 7 or less (e.g. p-toluenesulfonic acid) and blending the treated aluminum oxide in a polycarbonate resin, the molecular weight of the polycarbonate resin becomes decreased due to the insufficient modification of the aluminum oxide; the resulting resin composition cannot show sufficient melt fluidity during molding; and there may arise problems such as dissolution of metal components from a production apparatus (e.g. a biaxial extruder) by the metal corrosivity of the sulfonic acid having a carbon number of 7 or less.
Further, Patent Document 6 teaches a polycarbonate resin composition containing a composite material of alumina particles to which a phosphoric ester is chemically bonded. It has been found by the present inventors that the composite material of the alumina particles and the phosphoric ester has the problem that the phosphoric ester cannot be bonded stably to the alumina particles at a molding temperature of the resin composition (e.g. 250° C. or higher), thereby resulting in performance deteriorations of the resin composition (such as aggregation of the alumina particles and hydrolysis of the polycarbonate resin).
Patent Documents 7 and 8 teach polycarbonate resin compositions containing composite materials of alumina particles to which alkylbenzenesulfonic acids are chemically bonded. It has been found by the present inventors that: these polycarbonate resin compositions fail not only to prevent hydrolysis of the polycarbonate resin by the alumina particles, but to show improved melt fluidity due to the strong interaction between the alumina particles, as the amount of the alkylbenzenesulfonic acid used is so small that the alumina particles cannot be sufficiently coated with the alkylbenzenesulfonic acid.
As mentioned above, various researches have been made to improve the characteristics properties of the polycarbonate resin composition containing the aluminum oxide nanoparticles. However, there has not yet been developed any polycarbonate resin composition that combines high transparency and dimensional stability with excellent mechanical properties and moldability.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-054941    Patent Document 2: Japanese Laid-Open Patent Publication No. 2006-062905    Patent Document 3: Japanese Publication of International Patent Application No. 2005-528474    Patent Document 4: Japanese Laid-Open Patent Publication No. 2004-149687    Patent Document 5: Japanese Publication of International Patent Application No. 2003-517418    Patent Document 6: Japanese Laid-Open Patent Publication No. 2007-031684    Patent Document 7: Japanese Laid-Open Patent Publication No. 2007-002089    Patent Document 8: Japanese Laid-Open Patent Publication No. 2006-193400
It is therefore an object of the present invention to provide a polycarbonate resin composition containing aluminum oxide nanoparticles and being capable of showing good moldability and retention heat stability (hereinafter occasionally just referred to “heat stability”) during production and molding of the resin composition and combining high transparency and dimensional stability with excellent mechanical properties.